This invention relates generally to a printing apparatus and method for monochromatic imaging onto a photosensitive media by spatially and temporally modulating a light beam and more particularly to a film recording apparatus capable of forming a high-resolution monochromatic image on photosensitive media.
Conventional printers generally adapted to record images provided from digital data onto photosensitive media apply light exposure energy that may originate from a number of different sources and may be modulated in a number of different ways. In photoprocessing apparatus, for example, light exposure energy can be applied from a CRT-based printer. In a CRT-based printer, the digital data is used to modulate a Cathode Ray Tube (CRT), which provides exposure energy by scanning an electron beam of variable intensity along its phosphorescent screen. Alternately, light exposure energy can be applied from a laser-based printer, as is disclosed in U.S. Pat. No. 4,728,965 (Kessler et al.) In a laser-based printer, the digital data is used to modulate the duration of laser on-time or intensity as the beam is scanned by a rotating polygon onto the imaging plane.
CRT and laser-based printers perform satisfactorily for photoprocessing applications, that is, for printing of photographs for consumer and commercial markets. However, in an effort to reduce cost and complexity, alternative technologies have been considered for use in photoprocessing printers. Among suitable candidate technologies under development are two-dimensional spatial light modulators.
Two-dimensional spatial light modulators, such as those using a digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or using a liquid crystal device (LCD) can be used to modulate an incoming optical beam for imaging. A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting or by blocking transmission of incident light from a light source by affecting the polarization state of light. Polarization considerations are, therefore, important in the overall design of support optics for a spatial light modulator.
There are two basic types of spatial light modulators in current use. The first type developed was the transmission spatial light modulator, which, as its name implies, operates by selective transmission of an optical beam through individual array elements. The second type, a later development, is a reflective spatial light modulator. As its name implies, the reflective spatial light modulator, operates by selective reflection of an optical beam through individual array elements. A suitable example of an LCD reflective spatial light modulator relevant to this application utilizes an integrated CMOS backplane, allowing a small footprint and improved uniformity characteristics.
Conventionally, LCD spatial light modulators have been developed and employed for digital projection systems for image display, such as is disclosed in U.S. Pat. No. 5,325,137 (Konno et al.) and in miniaturized image display apparatus suitable for mounting within a helmet or supported by eyeglasses, as is disclosed in U.S. Pat. No. 5,808,800 (Handschy et al.) LCD projector and display designs in use typically employ one or more spatial light modulators, such as using one for each of the primary colors, as is disclosed in U.S. Pat. No. 5,743,610 (Yajima et al.)
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,325,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for printing by photoprocessing apparatus. Projectors are optimized to provide maximum luminous flux to a screen, which secondary emphasis placed on characteristics important in printing, such as contrast and resolution. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as xe2x80x9cforgivingxe2x80x9d to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. For this reason, there can be considerable complexity in optical systems for providing a uniform exposure energy for printing. Even more significant are differences in resolution requirements. Adapted for the human eye, projection and display systems are optimized for viewing at typical resolutions such as 72 dpi or less, for example. Photographic printing apparatus, on the other hand, must achieve much higher resolution, particularly apparatus designed for micrographics applications, which can be expected to provide 8,000 dpi for some systems. Thus, while LCD spatial light modulators can be used in a range of imaging applications for projection and display to high-resolution printing, the requirements on supporting optics can vary significantly.
Largely because spatial light modulators can offer significant advantages in cost and size, these devices have been proposed for different printing systems, from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748 (Sarraf), to area printing systems such as the system described in U.S. Pat. No. 5,652,661 (Gallipeau et al.) One approach, using a Texas Instruments DMD as shown in U.S. Pat. No. 5,461,411 offers advantages common to spatial light modulator printing such as longer exposure times using light emitting diodes as a source as shown in U.S. Pat. No. 5,504,514. However, DMD technology is very specific and not widely available. As a result, DMDs may be expensive and not easily scaleable to higher resolution requirements. The currently available resolution using DMDs is not sufficient for all printing needs. Furthermore, there is no clear technology path to increased resolution with DMDs.
A preferred approach for photoprocessing printers uses an LCD-based spatial light modulator. Liquid crystal modulators can be a low cost solution for applications requiring spatial light modulators. Photographic printers using commonly available LCD technology are disclosed in U.S. Pat. Nos. 5,652,661; 5,701,185 (Reiss et al.); and 5,745,156 (Federico et al.). Although the present invention primarily addresses use of LCD spatial light modulators, references to LCD in the subsequent description can be generalized, for the most part, to other types of spatial light modulators, such as the DMD noted above.
Primarily because of their early development for and association with screen projection of digital images, spatial light modulators have largely been adapted to continuous tone (contone) color imaging applications. Unlike other digital printing devices, such as the CRT and laser-based devices mentioned above that scan a beam in a two-dimensional pattern, spatial light modulators image one complete frame at a time. Using an LCD, the total exposure duration and overall exposure energy supplied for a frame can be varied as necessary in order to achieve the desired image density and to control media reciprocity characteristics. Advantageously, for photoprocessing applications, the capability for timing and intensity control of each individual pixel allows an LCD printer to provide grayscale imaging.
Most printer designs using LCD technology employ the LCD as a transmissive spatial light modulator, such as is disclosed in U.S. Pat. Nos. 5,652,661 and 5,701,185. However, the improved size and performance characteristics of reflective LCD arrays have made this technology a desirable alternative for conventional color photographic printing, as is disclosed in commonly-assigned copending U.S. Pat. application Ser. No. 09/197,328, filed Nov. 19, 1998, entitled xe2x80x9cReflective Liquid Crystal Modulator Based Printing System,xe2x80x9d by Ramanujan et al. As is described in the Ramanujan application, color photographic printing requires multiple color light sources applied in sequential fashion. The supporting illumination optics are required to handle broadband light sources, including use of a broadband beamsplitter cube. The optics system for such a printer must provide telecentric illumination for color printing applications. In summary, in the evolution of photoprocessing systems for film printing, as outlined above, it can be seen that the contone imaging requirements for color imaging are suitably met by employing LCD spatial light modulators as a solution.
Printing systems for micrographics or Computer-Output-Microfilm (COM) imaging, diagnostic imaging, and other specialized monochrome imaging applications present a number of unique challenges for optical systems. In the COM environment, images are archived for long-term storage and retrievability. Unlike conventional color photographic images, microfilm archives, for example, are intended to last for hundreds of years in some environments. This archival requirement has, in turn, driven a number of related requirements for image quality. For image reproduction quality, for example, one of the key expectations for micrographics applications is that all images stored on archival media will be written as high-contrast black and white images. Color film is not used as a medium for COM applications since it degrades much too quickly for archive purposes and is not capable of providing the needed resolution. Grayscale representation, meanwhile, has not been available for conventional micrographics printers. Certainly, bitonal representation is appropriate for storage of alphanumeric characters and for standard types of line drawings such as those used in engineering and utilities environments, for example. In order to record bitonal images onto photosensitive media, exposure energy applied by the printer is either on or off, to create high-contrast images without intermediate levels or grayscale representation.
In addition to the requirement for superb contrast there is a requirement for high resolution of COM output. COM images, for example, are routinely printed onto media at reductions of 40xc3x97 or more. Overall, micrographics media is designed to provide much higher resolution than conventional dye-based media provides for color photographic imaging. To provide high resolution, micrographics media employs a much smaller AgX grain size in its photosensitive emulsion. Optics components for COM systems are correspondingly designed to maximize resolution, more so than with optical components designed for conventional color photoprocessing apparatus.
Conventional COM printers have utilized both CRT and laser-based imaging optics with some success. However, there is room for improvement. For example, CRT-based printers for COM use, such as disclosed in U.S. Pat. No. 4,624,558 (Johnson) are relatively costly and can be bulky. Laser-based printers, such as disclosed in U.S. Pat. No. 4,777,514 (Theer et al.) present size and cost constraints and can be mechanically more complex, since the laser imaging system with its spinning polygon and beam-shaping optics must be designed specifically for the printer application. In addition, laser printers exhibit high-intensity reciprocity failure when used with conventional photosensitive media, thus necessitating the design of special media for COM use.
More recent technologies employed for COM imaging include use of linear light-emitting diode (LED) arrays, such as in the Model 4800 Document Archive Writer, manufactured by Eastman Kodak Company, Rochester, N.Y. Another alternative is use of a linear light-valve array, such as is disclosed in U.S. Pat. No. 5,030,970 (Rau et al.) However, with both LED arrays and linear light-valve arrays, COM writers continue to be relatively expensive, largely due to the cost of support components and the complexity of drive electronics. There is a long-felt need to lower cost and reduce size and complexity for COM devices, without sacrificing performance or robustness.
A disadvantage of most conventional COM printers has been the need to use aqueous (AgX) development techniques for microfilm media. Most conventional COM printers are not adaptable for use with dry-processed media. Able to be handled in room lighting conditions and free from chemical processing, plumbing, venting, and dark room requirements, dry-processed COM microfilm represents an environmentally advantageous solution for micrographics systems.
It is significant to observe that dry-processed microfilm requires higher levels of exposure energy than must be provided for conventional aqueous media. As a rough indicator of relative exposure levels used to obtain maximum density, for instance, dry-processed microfilm requires 200 ergs/cm2 (nominal) vs. 1 erg/cm2 (nominal) for conventional photographic color media. This high exposure level requirement is above the range that is provided by most conventional COM imaging optics. Until now, only laser-based imaging has been employed to print onto dry-processed microfilm. In response to environmental concerns, it is highly desirable that any alternative COM imaging technology that is developed be able to use dry-processed microfilm.
For reasons outlined in the above description, spatial light modulators have not been adapted for COM printing applications. Widely used for continuous tone color imaging, spatial light modulators have been overlooked for monochrome applications such as for COM and related diagnostic imaging. Grayscale capability, for which LCD spatial light modulators are readily adaptable, has not been pursued for COM applications due, in part, to long-standing customer expectations, which, in turn, are based on limitations of the conventional imaging technologies used. Optical systems employing LCD modulators for color printing address broadband imaging concerns that result in design complexity and performance trade-offs that make these systems less than ideal for COM imaging. For example, color imaging systems such as is disclosed in the Ramanujan application use sequential color frame imaging techniques, necessarily compromising full polychromatic capability for speed. Moreover, the relative resolution requirements for COM printing are an order of magnitude greater than that applied for standard color printing applications. Optical subsystems used in LCD modulator-based color printers make design compromises in order to handle multiple wavelengths, which adversely impacts resolution capability.
Significantly, existing LCD modulators adapted for frame-based color printing applications are designed to provide exposure levels sufficient for aqueous media only. Thus, direct adaptation of existing LCD optical systems designs would not allow a COM printer to take advantage of dry-process microfilm, with its potential support cost and environmental benefits.
To date, conventional COM printers using laser, CRT, or other technologies do not currently provide grayscale capability and are not optimized to provide this capability onto archival media. While grayscale imaging capability has not yet been a requirement of COM users, it is recognized that developments such as the growth of Internet applications and image transfer, image format standardization, and expanding overall use of grayscale images in documents indicate a likely demand of this capability.
Thus, it can be seen that there is a need for environmentally improved imaging solutions in COM environments, with additional requirements to reduce cost and complexity and to provide grayscale imaging potential.
It is an object of the present invention to provide a printing apparatus using a spatial light modulator for monochromatic imaging onto photosensitive media, where the printing apparatus is capable of grayscale image recording.
With the above object in mind, the present invention provides a printing apparatus for recording digital image data onto photosensitive media, the apparatus comprising:
(a) a monochromatic light source;
(b) a uniformizer for uniformizing a wavefront of light emitted from said light source;
(c) a polarizer for filtering light uniformized by said uniformizing to provide a polarized beam having a predetermined polarization state;
(d) a spatial light modulator having a plurality of individual elements capable of altering said polarization state of said polarized beam to provide an exposure beam for printing, a state of each of said elements controlled according to said digital image data;
(e) an optics assembly for directing said polarized beam to said modulator and said exposure beam from said spatial light modulator; and
(f) a lens assembly for directing said exposure beam onto said photosensitive medium.
According to an aspect of the present invention, light is passed through an uniformizer or integrator to provide a source of spatially uniform, monochromatic light for the printing apparatus. The monochromatic light is then polarized and passed through a beamsplitter, which directs a polarized light component onto a spatial light modulator. Individual array elements of the spatial light modulator, controlled according to digital image data, are turned on in order to modulate the polarization rotation of the incident light. Modulation for each pixel can be effected by controlling the level of the light from the light source, by control of the drive voltage to each individual pixel in the spatial light modulator, or by controlling the duration of on-time for each individual array element. The resulting light is then directed through a lens assembly to expose the photosensitive media.
Features of the invention include a polarizer for filtering the polarization of incident light and an exit polarizer for filtering the polarization of output light.
An advantage of the present invention is that it provides a low-cost alternative to existing print methods. Spatial light modulators are commodity components that can be readily adapted in order to fabricate a printing apparatus for photosensitive media.
A further advantage of the present invention is that it provides the capability for grayscale imaging onto COM media.
A further advantage of the present invention is that it provides a simplified design particularly suited for monochrome imaging onto photosensitive media, without the requirements for broadband components and coatings and without concerns for telecentricity as is required for polychromatic imaging.
A further advantage of the present invention is that it allows the use of existing media used for earlier COM printing systems. Thus, introduction of a printer of the present invention is possible without the need to develop a new media having the appropriate response characteristics.
A further advantage of the present invention is that it provides an imaging apparatus that is capable of imaging onto dry-process COM media, with resulting environmental benefits.